The disclosed invention relates to distance measuring devices, and more particularly to devices which automatically measure distance by transmitting light beams to a target and making calculations based on reflections of the light beam from the target. One prior art device which makes such measurements, operates by transmitting a plurality of light pulses which are spaced apart in time by an interval which is larger than the time required for a single pulse to be transmitted to the target and reflected back to the device. These are known as time measurement devices, as opposed to phase measurement devices. In operation, a timing counter is started upon the transmission of the light pulse, and is stopped upon receipt of the corresponding relfected light pulse. A problem with this type device however, is that their operation is relatively slow. This is because the occurrence in time of the leading edge of the received pulse is difficult to determine with a single measurement. Accordingly, a large number of pulses, typically several hundred pulses, must be transmitted and received; and the time delay for each of these pulses is averaged to determine with suitable statistical confidence, the distance between the device and the target.
Other prior art devices operate on phase measurement principles. These devices transmit a light beam which is amplitude modulated by a particular frequency, and simultaneously receive reflections of the light beam from the target. In these devices, distance measurements are made based on the phase angle between the transmitted light beam and the reflected light beam. A problem with these devices however, is that in order for there to be no ambiguities in the phase angle, the modulating frequency must be low--such that the corresponding wavelength is greater than the distance to be measured. Typically, distances of 2,000 meters are to be measured; and thus the modulating frequency must typically be in the order of 75 KHz. Typically, the phase angle is determined by enabling a counter a count clock pulses of a fixed frequency at the zero degrees point on the transmitted light beam and by stopping the counter at the zero degrees point on the recieved light beam. Each count of the counter then represents a portion of the wavelength of the modulating frequency. However, since the wavelength must necessarily be larger than the distance that is measured, the size of the counter must also be large. And typically, to have each count of the counter represent a small increment of distance, such as a millimeter, would make the counter prohibitively large.
In order to overcome this resolution problem, more modern distance measuring devices which operate on phase measurement principles, typically transmit three frequencies. One of these frequencies has a wavelength larger than the total distance to be measured; while the second and third frequencies have wavelength which are fractions of the first frequency. In operation, phase measurements were made at each of the three frequencies. Then the low frequency is utilized to approximate the distance to the target, while the high frequencies are utilized to improve the resolution. A problem with these devices however, is that in order to generate short, medium and long wavelengths, the frequency spread of the light modulating signal is very large. For example, the frequencies typically ranged from 75 KHz to 30 MHz if the total distance was 2,000 meters and the resolution was 1 millimeter. Thus, these devices required sophisticated frequency generating circuitry that could modulate a light beam over such a widely varying range.
Also in the prior art, some devices automatically calculate the horizontal and vertical distances between the device and the target. These calculations depend upon the determination of the vertical angle formed by the target, the device, and the horizontal axis. In the prior art, this angle is determined by electro-mechanical angle sensing devices having resolution which is less than desired. Further, some of these devices are inoperable over the full vertical angle range of 0 to 90 degrees. Also, many of the devices have offset errors, are sensitive to temperature changes, to gravitation changes, and to component tolerances.
A variety of other problems also existed in the prior art devices. For example, after the reflected waveform is received in the device, and is converted to an electrical signal, the signal typically undergoes some processing before actual phase measurements are made thereon. For example, the signal may be mixed with other frequencies to obtain a frequency shift. This processing typically stretches or shrinks the signal, and this distortion introduces errors into the phase measurements.
Also, in order for the distance measuring device to measure both short distances and long distances, it must be capable of generating electrical signals representative of a reflected light beam which varies substantially in intensity. For example, the intensity of a light beam reflected from a close target may be as much as 80 dB times stronger than the intensity of a light beam reflected from a distant target. Accordingly, circuitry must be included in the device to compensate for this varying signal strength. In the past, this compensation was performed exclusively by electronic circuits, and these are both expensive and complex.
Accordingly, it is one object of the invention to provide an improved distance measuring device.
Another object of the invention is to provide a distance measuring device which transmits only high frequency signals.
Another object of the invention is to provide a distance measuring device having an improved vertical angle sensor.
Another object of the invention is to provide a distance measuring device having in field calibration that compensates for substantially all offset errors.
Another object of the invention is to provide a distance measuring device which compensates for phase errors introduced by signal processing distortion.
Still another object of the invention is to provide a distance measuring device which mechanically compensates for intensity variation in the reflected light.